1. Field of the Invention
The invention relates to an abnormality diagnosis apparatus and an abnormality diagnosis method for an internal combustion engine, and more particularly to an abnormality diagnosis apparatus and an abnormality diagnosis method for diagnosing an abnormality on the basis of a rotation variation amount of an output shaft of an internal combustion engine provided with a plurality of variable valve timing (VVT) mechanisms.
2. Description of the Related Art
An internal combustion engine provided with a plurality of cylinders is available. Typically, control is performed such that all of the cylinders have an identical air-fuel ratio. However, when the internal combustion engine is operated over a long time period, differences may occur among the respective air-fuel ratios of the cylinders due to deterioration of an injector provided in each cylinder, individual differences, and so on. When differences occur among the respective air-fuel ratios of the cylinders, differences may occur in a torque obtained in a combustion stroke of each cylinder. Hence, in one proposed method, rotation variation in an output shaft of the internal combustion engine is used to detect an abnormality according to which the cylinders have different air-fuel ratios.
Japanese Patent Application Publication No. 2010-24977 (JP-A-2010-24977) discloses a diagnosis control apparatus for an internal combustion engine that controls an amount of fuel injected into each cylinder and diagnoses the presence and extent of air-fuel ratio or output variation among the cylinders. The diagnosis control apparatus measures a time required for a crankshaft to rotate by a predetermined angle in relation to each cylinder, extracts a 0.5 order component serving as a rotation variation component for each two revolutions of the crankshaft or a 1.0 order component serving as a rotation variation component for each single revolution of the crankshaft in relation to each cylinder on the basis of the measured required time, counts the number of times the 0.5 order component or the 1.0 order component deviates from a set range within a set time period in relation to each cylinder, and diagnoses an abnormality in the air-fuel ratio or output of the corresponding cylinder when the count value exceeds a predetermined value.
In a V type engine or a horizontally opposed engine provided with a VVT mechanism that modifies an opening/closing phase (a crank angle) of an intake valve and an exhaust valve in accordance with operating conditions, a first bank is provided with a separate VVT mechanism to a VVT mechanism provided in a second bank. Therefore, even when the internal combustion engine is normal, the phase of the valves in the first bank and the phase of the valves in the second bank may differ within an allowable range. Even in this case, air-fuel ratio variation may occur between the cylinders of the first bank and the cylinders of the second bank. Hence, when the presence of an abnormality is diagnosed on the basis of the amount of rotation variation in the output shaft of the engine, an abnormality may be detected erroneously.